I converted my gravel bike to a single speeder last July 2020. But let's backtrack a bit more.
Where I lived, SS conversion kit wasn't easy to find. In fact I didn't found any. I've scoured a dozen LBS for a couple of months and still couldn't find one and it's not even COVID related. I did came across one Fixed Gear shop but they only got fixed gear specific. So that's also out. I've contacted some bike shops in another city, they're out of stock (COVID related). And US-based shop have shipping difficulties due to COVID. Short story, Ali-Express to the rescue. Cost me just under US$20 per SS Kit. Comes with spacers, cog & lockring. You can buy their chain tensioner with a kit or separately. I didn't get the tensioner 'coz my frame had a horizontal dropout. In comparison, Surly, my first option was $40 spacer & lockring set, $30 cassette cog. I got 3 ZTTO kits, 16T, 18T, 20T. I wasn't sure if I will be good with a 16T or 18T but the 20T was for the mtb, which will probably be later this year. So 3 ZTTO kits for the price of 1 Surly kit... it's a no brainer even if stocks were available. At the time I got it, it only came in purple. And I eventually spend more purplelizing (if that's even a word) my bike.
The review... I've installed the 18T and you'll be happy to know that I got around 3,000km on it. I've paired it with 3 chainrings, depending on my rides. A 40T round and a couple of ovals, 36T & 42T. I've done gravel rides, muddy rides, roads (a few century & a double century) and even some light trails. Got rained on and even rode beside the sea. And I think you all know when the hills come, single speeders are gonna have to mashed it up. Suffice to say, it was ABUSED. Pictures 1-3 is the unused 16T and Picture 4 is the abused 18T. Doesn't look abused... do I need say more? That one 3,000km picture is enough to say it is worth more than what it costs.
And it now comes in red. Oh, btw, they are aluminum.
